improve UX for file targets that do not write cracks to potfile

[roycewilliams]

Reference thread: https://hashcat.net/forum/thread-9490-post-50261.html#pid50261

A couple of suggestions:

• Consider restoring writing to potfile specifically for the formats that are filename-based

• If that's not attractive, consider generating a warning-level message to the user saying something like "WARNING: This format does not write cracks to the potfile - use -o" or something like that

[roycewilliams]

Just curious - what is the reason for not writing to the potfile? The previous behavior was simple and convenient.

[Chick3nman]

I believe it's to do with colliding entries in the potfile causing you to have to disable the potfile anyway if the entry is not a match. We throw other warnings like the warning for --keep-guessing about that dangers of duplicate entries in the potfile.